What is the simplest way of extracting the depth values of the scene?
I'm not sure I understood the request.
In case you intend to as depth values: values of the z-buffer of the camera
then the answer is easy.

you have to use this simple code:

#first import the libraries
import bpy
from bpy_extras.object_utils import world_to_camera_view

context = bpy.context
scene = bpy.context.scene   

#create the def for take the z-buffer
def checkCameraView(cam, obj):
    for v in obj.data.vertices:
        x, y, z = world_to_camera_view(context.scene, cam, obj.matrix_world * v.co)
        print(z)

#select the camera
cam = scene.camera
#check every mesh type objects in the scene
for obj in scene.objects:
    if obj.type == 'MESH' :
        checkCameraView(cam, obj)


z is the depth of the vertex
x and y are the coordinates on the Cartesian coordinate plane (2D) of the chamber
x = horizontal
y = vertical

api link: world_to_camera_view

This was the question?